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I've owned both the Xciting 500 and Xciting 250 at the same time.
You can't really tell the difference between the two bikes until you get past 55mph, they are pretty much identical before that.   The 250 really starts losing oomf at around 60 and struggles to reach 70, but the 500 will keep on going up to 90 (which is the fastest I took mine, well into the red line @10K rpm, but I could tell there was a little more available if I was willing to risk the engine).  They are both geared to have their powerband in highway speeds.

The quick check is to pop open the seat and look at the underseat storage.  If there is a carburetor access hatch and an air filter access hatch under the seat, it's a 500.  If you see no such ports its a 250.  Your statement about not wanting to get past 70mph leads me to believe you were driving a 250.  If it is a 500, it is definitely having issues.

I had to replace the stator on both bikes (and replaced the regulator and battery at the same time as insurance).

The OEM Kymco belts are very, very good.

3 years ago, I purchased a 2006 XC250 which had 3600 miles on it.  The owner had garaged it for almost 9 years, so as part fo the initial workup after towing it home, I replaced the belt with an OEM belt, even though the original one looked fine (rubber parts and aging, after all).

The bike now has almost 17,000 miles on it.  I check the belt each time I change the oil (1,200 miles), and it still looks fine and I haven't replaced it since.  Keep in mind I weigh 260 lbs. and have a 25 mile commute each way to work - part of it is an uphill climb, and 18 of those miles is typically highway (speeds up to 70mph), both of these place stress on the belt.

My advice would also be to check the belt, and if it looks good, don't bother with packing a spare.  I'd be far more concerned with the possibility of a flat caused by hitting a nail.

Also, if you're consistently replacing damaged belts under 10,000 miles, you either didn't use an OEM belt, or have drivetrain problems (most likely damage to the surfaces the belt rides on, but possibly alignment issues).

EVERYONE who isn't (the "indestructible" teenager who just got out of MSF classes and decided that SLPR doesn't really need to be followed because ITS ALL ABOUT GOING FAST, NOT GETTING THERE ALIVE) slows down before entering any curve where, due to the traffic or roadside obstructions, you can't see 3-4 seconds ahead. 

Spongy brake and no evidence of leaking brake fluid = air in the hydraulic system.
The mechanic who replaced it didn't fully bleed the brake line.
I bleed at the caliper until no air comes out (this is awkward using a mightyvac because the caliper and brake handle are on oppposite sides of a bike, much quicker to use two people).
Then remove ALMOST ALL the fluid from the reservoir and compress the caliper piston (with a C clamp) all the way down (removes any air trapped near the top of the brake lines).
Then re-attach caliper, add brake fluid to reservoir, and pump brake, adding fluid to reservoir after every few pumps to make sure that you DONT run out while pumping, until the brake pads seat.  Top off fluid and close up shop.

As a side note, on my Xciting 250, which has linked front/rear brakes on the left brake handle, I have *never* been able to get the left brake handle to feel like the right (front brake) one, it seems like it just needs more travel to put the same amount of force on the brakes.  I spent almost an hour bleeding it (across two days) at the calipers, the distribution relay, and the reservoir.  It isn't "squishy" in the sense that it doesn't "firm up" after a few pumps, its always like that, so I think I don't have air in the system, it just works this way.

I've owned the Xciting 500 (which I sold), and still own an Xciting 250 (which I have put over 16,000 miles on). 

They are both VERY stable on the highway.  The Xciting frame is Kymco's highway bike, you have the double disc brakes up front, a decent amount of mass, and the bodywork is such that it doesn't get blown around with crosswinds like my Forza (which is somewhat worse than the xciting) or old Reflex clone (which would require counter steering in any strong breeze).  Mileage on the XC500 wasn't exactly great (42mpg), but it DID have adequate highway power, 85 mph was not a problem (engine not even in redline yet).  The XC400 has the same power but loses a couple of pounds, should be a fantastic highway/touring bike, but with better mileage, just don't expect it to be quite as nimble around town as your 300, since you have the additional weight.  Also, the stock windshield on the 400, being a clone of the windshield that they've been putting on the Xciting since 2009, isn't very good, you'll get a blast of wind on your helmet unless you are riding in crotch rocket position.  Not nearly as bad as the stock Forza windshield (which is decorative and doesn't actually protect you from the wind at all), but you'll probably want to replace it quickly.

The Xcitings tended to lite up the tank empty warning light at around 130 miles, but could probably go another 30 miles beyond that.  It didn't bother me much when I did extensive highway travel, since I needed to take a break every hour or so; I would refuel every other break.

Check the manual for the oil service interval!  The XC500 has an oil filter and a 4000mile oil service interval.  The XC250 has a screen (no filter) but a 1200 mile service interval.  (The Honda Forza has an 8000 mile oil service interval.)  I have no idea what it is on the XC400, but if its 1200 miles, and you do a lot of riding, you will be changing oil once a month.

Finally, underseat storage on the XC500 was *not* very good.  Make sure you pop the seat on the XC400 before putting down the cash, because it is possible you might be wanting a storage box as well.

Per the XC500 owner's manual, inspect every 8,000 miles, and change at 24,000 miles.  So, extract some and visually inspect it for contamination - if it looks ok, good for another 8,000 miles, if it looks contaminated, check the water pump seals (or the cylinder gaskets) and replace.

Inspect belt.  Inspect clutch shoes, buff with a brass brush if glazed over, replace them all if one is cracked.   Clean up inside of bell housing with brake cleaner.  If you have the tools, disassemble the rear pulley and re-grease the sliding shafts and bearings.  Do not attempt to clean the pulley belt contact surfaces with cleaner or grease cutter unless you are sure that it won't drip down onto the drive shaft, and that you can clean it all off afterwards (i.e. you have the pulley already disassembled).  Do NOT overlubricate, since it will fly outwards and potentially onto the belt or belt friction surfaces.

While you have the case open and the belt off, it wouldn't hurt to clean up and relubricate the variator as well.